Glazed electric range heating unit and glaze therefor



J. D. MORGAN July 2, 1935.

GLAZED ELECTRIC RANGE HEATING UNIT AND GLAZE THEREFOR Filed Oct. 17, 1931 INVENTOR JOHN D. MORGAN A TTORNEY Patented July 2, 1935 GLAZED ELECTRIC RANGE HEATDIG UNIT AND GLAZE THEREFOR John D. Morgan, South Orange, N. J., assignor to Doherty Research Company, New York, N. Y., a corporation of Delaware Application October 17, 1931, Serial No. 569,552

-3 Claims.

This invention relates to ceramic glazes, and more particularly it concerns glazes adapted for use in connection with electric heating units intended for domestic purposes wherein various 5 cooking utensils come in direct contact with such units. It is of especial utility in connection with electric range heating units of the type disclosed in my copending patent application Ser. No. 541,227 filed June 1, 1931, wherein the work- .carrying member, which has imbedded therein the electric resistor element, is made of molded refractory material of high heat conductivity.

The electric heating unit disclosed in my aforementioned co-pending application comprises at least two members or plates of refractory material, preferably intimately secured together by suitable bonding means. The top plate, which houses the electric resistor element and is intended to support the object to be heated, has a high thermal conductivity substantially greater than that of silicon carbide. It is made of refractory mixture containing silicon, with or Without zircon and silicon carbide, bonded by a compound such as orthophosphoric acid, non-volatile at 2600 F.

The bottom plate of the said heating unit is made of a porous white or light-colored refractory having a very low conductivity and adapted to facilitate reflection of heat flowing thereto from the top member, and which is capable of reradiating to the upper plate most of the heat transmitted thereto through the latter. The light-colored plate is made of a suitable refractory aggregate, a small amount of finely divided metal, and a bonding agent capable of generating a gas when in contact with the said metal. An example of such composition is a mixture of native zirconium sand, approximately 1% of its weight of aluminum powder; 15% of rosin and 6% of P205, half of the latter preferably being in the form of HaPO4 and half in the form of ammonium acid phosphate in aqueous solution. Each of these refractory elements or plates is baked or furnaced at temperatures in the neighborhood of 2000 F., or at temperatures somewhat above those at which the refractory is subsequently to be employed.

Surface glazes for various kinds of electric heating units including those having a refractory work-carrying surface are already known to those skilled in the art. However, the glazes heretofore employed for the purpose have been found unsatisfactory in service since they break down in use and fail to properly protect the refractory body and the electric resistor element embedded therein, the latter of which, as is well known, is

subject to rapid deterioration at high temperatures in the presence of air and various other gases. The usual types of glaze employed hereto fore for glazing have fusing points ranging up to somewhat less than 1500 F., and furthermore,

they start to soften long before they reach that state of fusion where they properly flow and can be properly applied to the work-carrying surface of the heating unit. Such glazes have some utility in connection with heating units designed to operate well below the above-mentioned temperatures.

However, many series of tests conducted in connection with a wide variety of electric heating units have shown that for effective use of the electric current for heating purposes, heating element temperatures of approximately 1800 F. to 2000 F. are required. Furthermore, where certain cooking utensils are employed, (particularly those made of aluminum), which have a pronounced concave contour on their bottom surface disposed next to the heating unit, the concavity serves to form a pocket for dead air which is not free to circulate,-with the result that the glazed surface of the electric range heating unit immediately below this air pocket reaches and is maintained at an abnormally high temperature well above that at which the balance of the surface of the heating is maintained-temperatures of the order of 1450 F. to 1550 F. having been observed. As a result the usual commercial glazes blister and roughen and eventually the glaze fails. Thereafter any materials falling onto the surface of the unit may penetrate the refractory aggregate and may carbonize therein or react with the metals of the resistor element at the high temperature attained, seriously interfering with the heating efficiency of the resistor element and shortening its life.

It has been determined that this injury to the usual commercial glazes is due in considerable degree to the long range softening period which glazes undergo before flowing or melting.

The most desirable glaze for electric heating elements of the type wherein the electric resistor element is embedded within a refractory workcarrying member is one which will not soften at temperatures of the order of 1600 F. and yet will be perfectly cured and free-flowing at temperatures belowthe maximum operating temperatures of the resistor element,which is around 2000 F.

Among the principal objects of the present invention are: to provide an electric range heating unit having a protective glaze adapted to resist softening at temperatures up to 1600 F., but which is liquid at temperatures of the order of 2000" F., to provide an electric range heating unit having a high-melting surface glaze which will, however, melt and flow at temperatures substantially below that at which the electric resistor element is injured by exposure to air; and to provide a glaze for ceramics which will not soften below 1550" F., but which is free-flowing and fully cured at temperatures below 2100 F.

In its preferred form the glaze forming the subject matter of the invention comprises a mixture of potash, lime, alumina, iron oxide, silica and boric oxide, or their equivalents in the following proportions:

.3 parts by weight of potash .7 parts by weight of lime .3 parts by weight of alumina .2 parts by weight of iron oxide 3.5 to 3.75 parts of silica .45 to .25 parts of boric oxide (B203) This mixture is melted in an oxidizing atmosphere within a furnace or leer at temperatures around 2100 to 2300 F. for a period of time adapted to permit interaction of the ingredients which may for example require three hours. The melt is then cooled in well-known manner and is finely ground.

The iron oxide component of the glaze tends to produce a glaze having a slight coloration, which for some purposes may be objectionable. Where such is the case, the iron oxide may be replaced by an equivalent amount of zinc oxide; and this will give a glaze having the desired physical properties-here described.

An aqueous suspension of the finely ground glaze material is sprayed over the surface of the refractory plate having embedded therein the electric resistance element. The thus-coated refractory plate or element is then placed in an oxidizing atmosphere in a furnace and is brought to a temperature of around 2000" F., after which it is permitted to cool slowly in air.

If it is desired to color the glaze, without however reducing the desired high-softening temperature possessed by the uncolored glaze, small amounts of oxides adapted to provide the desired color are added to the glaze before the latter is sprayed upon the refractory plate; or, alternatively, an aqueous suspension of such colorimparting oxides may first be sprayed upon the refractory plate, following which, after evaporation of the water, the spray of high-softening point uncolored glaze is applied over the other coat, and the coated plate is fired in the manner described above.

Small amounts of manganese and iron oxide, or cobalt and iron oxide when added to the glaze give a black color; cobalt oxide imparts a blue color; chromium or nickel oxide gives a green color; and potassium bichroi nate and alumina give a red color to the high softening point glaze described. Various other color combinations of course may be eifected by varying the amount of the above-mentioned oxides or by combining various of these color-imparting oxides, together, or with other oxides.

Where it is desired to produce an electric range heating unit having a glaze with an initial softening temperature of 1650 F. the above-mentioned glaze formula employing 3.5 parts of silica and .45 parts of boric oxide is employed; while if it is desired to produce a glaze having an initial softening temperature of around 1830 F., 3.75 parts of silica and .25 parts of boric oxide are employed in the manufacture of the glaze. Likewise, glazes having still higher initial softening temperatures may be produced by somewhat further increasing the percentage of silica and/or reducing the amount of boric oxide employed.

The spray of glaze-forming materials may be applied to the electric-resistor-carrying refractory member immediately subsequent to its formation and before it has been submitted to any heat treating operation; or alternatively, after formation of the refractory plate the latter may be fired to any suitable temperature prior to spraying thereon the glaze-forming coating. In the former case the high temperature firing of the refractory plate, the melting of the glaze, and the thorough bonding thereof with the said plate are simultaneously effected.

In the attached drawing showing a unitary electric heater embodying the present invention, Fig. 1 is a vertical section through one form of such heater, and Figs. 2 and 3 are vertical sections through certain modified constructions.

In the drawing, l designates a utensil-carrying plate of refractory material having a high thermal conductivity, preferably greater than that of silicon carbide, and of the composition heretofore disclosed. The said plate forms the closed top of an electric range heating unit,-and its lower surface is provided with a spiral groove or grooves l2 adapted to house the usual electric resistor element M. The top member I0 is secured upon the upper surface of a plate 20 of porous refractory material of low heat conductivity, already described. The terminals of the electric resistor I4 extend through suitable apertures 22 formed in the porous refractory plate 20 and are connected in an electric circuit 24. A relatively thin coating or layer of a glaze 36 of the nature heretofore described covers at least the upper or article-carrying surface of the plate I0.

In the modification shown in Fig, 2, a metal resistor wire or ribbon 32 is located in the plate ill at a substantia1distance above the porous refractory plate 20, and preferably near the upper surface of the top plate I0. The coating of glaze 36 may be applied to the surface of the said plate l0 subsequent to its formation and before it has been exposed to any heat treatment. In such case the glaze is then melted and thoroughly bonded with the plate at the time of the firing of the latter.

According to the modification shown in Fig. 3, each of a pair of members or plates in is mounted on one of the respective sides of the porous refractory plate 20. The resist r elements I4 may be mounted in spiral grooves in each of the outer plates ID as shown or, in lieu thereof ,'the elements l4 may be mounted in grooves in the oposite surfaces of the porous member 20. Such a construction is employed to advantage in an installation requiring the simultaneous use of the top member In for baking and the bottom member ill for grilling. The exposed surfaces of the top member l0, and preferably also of the bottom member ID are provided with thin layers of the glaze 36.

It will be obvious to those skilled in the art that the novel glaze described herein may be employed for many uses in addition to that recited. The invention is susceptible of modification within the scope of the appended claims.

I claim:

1. In an electrical heating unit comprising a refractory work carrying member characterized by high thermal conductivity and low electrical conductivity wherein an electric resistor element is embedded in the refractory member which element is subject to rapid deterioration upon prolonged exposure to temperatures of 2100 degrees or above, the combination of said refractory work carrying member composed of silicon and zircon bonded with a phosphate reaction product formed by firing said mixture with a phosphoric acid binder, and a glaze comprising the heat reaction products of silicon, aluminum, iron oxide, lime, potash and boric acid, united with said member by heat fusion, said glaze having a glass-like appearance with a softening point not lower than 1550 F. and still being soft and free-flowing and thoroughly cured below 2100 F.

2. In an electrical heating unit comprising a refractory work carrying member characterized by high thermal conductivity and low electrical "said member comprising a mixture of .3 parts by weight of potash, .7 parts by weight of lime, .3 parts by weight of aluminum, .2 parts by weight of iron oxid, 3.5 to 3.75 parts by weight of silicon and .45 to .25 parts of boric acid, said glaze having a softening point not lower than 1550 F. and being free-flowing and fully cured at temperatures below 2100 F.

3. In an electrical heating unit comprising a refractory work carrying member characterized by high thermal conductivity and low electrical conductivity wherein an electric resistor element is embedded in the refractory member which element is subject to rapid deterioration upon prolonged exposure to temperatures of 2100 degrees or above, the combination of said refractory work carrying member composed of silicon and zircon bonded with aphosphate reaction. product formed by firing said mixture with a phosphoric acid binder, and a glaze united with said member comprising a mixture of .3 parts by weight of potash, .7 parts by weight of lime, .3 parts by weight af aluminum, .2 parts by weight of iron oxid, 3.5 to 3.75 parts by weight of silicon and .45

to .25 parts by weight of boric acid, a small JOHN D. MORGAN. 

